Abstract
The male-specific region of the human Y chromosome (MSY) is characterized by the lack of meiotic recombination and it has long been considered an evolutionary independent region of the human genome. In recent years, however, the idea that human MSY did not have an independent evolutionary history begun to emerge with the discovery that inter-chromosomal gene conversion (ICGC) can modulate the genetic diversity of some portions of this genomic region. Despite the study of the dynamics of this molecular mechanism in humans is still in its infancy, some peculiar features and consequences of it can be summarized. The main effect of ICGC is to increase the allelic diversity of MSY by generating a significant excess of clustered single nucleotide polymorphisms (SNPs) (defined as groups of two or more SNPs occurring in close proximity and on the same branch of the Y phylogeny). On the human MSY, 13 inter-chromosomal gene conversion hotspots (GCHs) have been identified so far, involving donor sequences mainly from the X-chromosome and, to a lesser extent, from autosomes. Most of the GCHs are evolutionary conserved and overlap with regions involved in aberrant X–Y crossing-over. This review mainly focuses on the dynamics and the current knowledge concerning the recombinational landscape of the human MSY in the form of ICGC, on how this molecular mechanism may influence the evolution of the MSY, and on how it could affect the information enclosed within a genomic region which, until recently, appeared to be an evolutionary independent unit.
Highlights
Human sex chromosomes originated from a single pair of ancestral recombining autosomes that began to differentiate between 160 and 190 million years ago (Mya), after the split of monotremes from Theria (Luo et al, 2011; Katsura et al, 2012)
With the exclusion of ancient episodic X–Y gene conversion events (Pecon Slattery et al, 2000; Iwase et al, 2010), which occurred during sex chromosome evolution, and very rare illegitimate crossing over events which have generated chromosomal aberrations (Vollrath et al, 1992; Schiebel et al, 1997), the MSY has long been considered an evolutionary independent region of the human genome
This view has been recently dismissed by the discovery that the sequence landscape of the human MSY can be modulated by inter-chromosomal gene conversion (ICGC) which may occur in narrow portions of the X-degenerate region called gene conversion hotspots (GCHs) (Rosser et al, 2009; Cruciani et al, 2010; Trombetta et al, 2010, 2014, 2016; Niederstätter et al, 2013)
Summary
Human sex chromosomes (the X and Y chromosome) originated from a single pair of ancestral recombining autosomes (proto-sex chromosomes) that began to differentiate between 160 and 190 million years ago (Mya), after the split of monotremes from Theria (Luo et al, 2011; Katsura et al, 2012). With the exclusion of ancient episodic X–Y gene conversion events (Pecon Slattery et al, 2000; Iwase et al, 2010), which occurred during sex chromosome evolution, and very rare illegitimate crossing over events which have generated chromosomal aberrations (Vollrath et al, 1992; Schiebel et al, 1997), the MSY has long been considered an evolutionary independent region of the human genome This view has been recently dismissed by the discovery that the sequence landscape of the human MSY can be modulated by inter-chromosomal gene conversion (ICGC) which may occur in narrow portions of the X-degenerate region called gene conversion hotspots (GCHs) (Rosser et al, 2009; Cruciani et al, 2010; Trombetta et al, 2010, 2014, 2016; Niederstätter et al, 2013). The average X–Y nucleotide identity in stratum 4 is about 87% (due to the stop in recombination which occurred about 40 Mya; Skaletsky et al, 2003), whereas the sequence similarity for VCX/VCY genes is above 98% (Ross et al, 2005)
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